Most currently available sodium/sulfur cells utilize a chrome-plated stainless steel cathode container. Although chrome is a corrosion-resistant material, it has been found that chrome exhibits slow-rate corrosion in the hostile, highly-corrosive environment of sodium/sulfur (Na/S) cells. More particularly, presently available Na/S cells utilizing a chrome-plated stainless steel cathode container have typically only achieved cycle lives of 2,000 to 6,000 cycles in laboratory testing, and further, have exhibited degradation of performance of an extent which significantly limits the longevity or effective useful life of these Na/S cells to approximately 1,500 to 2,000 cycles. Because of these shortcomings, presently available Na/S cells are considered undesirable for certain applications, e.g., spacecraft applications, for which an elongated effective useful life and high performance are critical. For example, most satellite applications require cells or batteries which have a minimum 10-year service life over thousands (e.g., 30,000) of cycles, with little or no degradation in performance.
Over the past 20 years, the problem of cathode container corrosion in Na/S cells has been studied extensively, since cathode container corrosion has long been identified as a primary cause of cell performance degradation and, in some cases, cell failure. Regarding the use of chrome-plated, stainless steel cathode containers in Na/S cells, it has been discovered that there are two primary mechanisms responsible for the performance-degrading corrosion of these cathode containers. The first primary mechanism is sulfur/sodium polysulfide attack of the chrome coating which results in the formation of chrome sulfides which eventually spall off, thereby exposing portions of the stainless steel substrate/support structure. The chrome sulfides, in conjunction with iron and nickel sulfides which are formed when the stainless steel is exposed, greatly increase the cell impedance, thereby degrading cell performance commensurately. The second primary mechanism is formation of sodium thiochromate (NaCrS.sub.2), which contaminates the electrolyte, which in turn, can cause cell failure. Nevertheless, although a great number of other materials have been evaluated for minimizing cathode container corrosion in Na/S cells, chrome is still generally considered to afford the best protection (at a feasible cost) against the highly corrosive, hostile environment present in Na/S cells, and thus, is the material currently employed as the primary form of corrosion protection in Na/S cells. Amongst the many other materials which have been evaluated for minimizing cathode container corrosion are transitional metal sulphides (e.g., iron sulfides), transitional metal carbides and nitrides, and graphite.
Although molybdenum and especially graphite have been found to exhibit superior resistance to sulfur and sodium polysulfide corrosion, and the requisite electrical characteristics, no feasible or effective technique for incorporating graphite and/or molybdenum within a cathode container structure has yet been derived or envisaged. For example, one approach, which is disclosed in U.S. Pat. Nos. 4,492,021 and 4,568,620, both issued to Wright et al., has been to isostatically press exfoliated graphite flake or grafoil (graphite foil) onto the interior surface of a metal substrate comprising the outer layer of a composite cathode container structure. This approach has resulted in an uneven graphite coating having marginal adhesion to the metal substrate, which has proven to be an ineffective long-term corrosion protection mechanism for the cathode container. Other approaches which have proven to be ineffective and/or infeasible for many cell applications, are taught in U.S. Pat. Nos. 3,959,013, issued to Breiter; 4,460,662, issues to Damrow et al.; and, 4,166,156 issued to Ludwig.
More particularly, the Breiter patent discloses a cathode container (for an Na/S cell) comprised of a metal substrate constituted of aluminum, steel, or iron-nickel-cobalt alloys having a layer of molybdenum or graphite adhered to the inner surface thereof. This proposed cathode container structure suffers from the following two major disadvantages:
(1) neither molybdenum nor graphite alone provide reliable, long-term corrosion protection for the container without unduly degrading cell performance; and,
(2) the adhesive bond between either molybdenum or graphite and any of the above-delineated metal substrate materials is not sufficiently strong to withstand long-term thermal and mechanical loadings, particularly loadings of the type and magnitude experienced in spacecraft applications, thus rendering the container structure unduly susceptible to delamination and/or other types of structural flaws attributable to insufficient adhesion of the corrosion-resistant layer to the underlying metal substrate.
Of course, these flaws result in cell performance degradation and possibly cell failure due to exposure of the metal substrate to the cathodic reactants contained within the cell. This problem of insufficient adhesion of the corrosion-resistant material to the metal substrate material is at least partially attributable to a mismatch between the coefficients of thermal expansion of these materials.
The Damrow et al. patent attempts to solve the above-identified problems associated with the Breiter container by means of coating the molybdenum protective overlayer with a surface layer of a metal oxide, e.g., MoO.sub.2. However, this container construction does not eliminate the previously discussed problem of insufficient adhesive bonding between the molybdenum and the metal substrate (e.g., aluminum), and therefore, although the Damrow et al. cathode container does exhibit better corrosion resistance than the Breiter cathode container, it does not provide the level of long-term corrosion resistance without performance degradation required for many applications, e.g., spacecraft applications.
The cathode container disclosed in the Ludwig patent, in one embodiment thereof, comprises a standalone graphite tube having a pyrolitic graphite coating deposited on its inner surface which is exposed to the cathodic reactant of the Na/S cell in which it is employed. Although this container structure provides superior corrosion resistance, it does not provide the level of corrosion resistance which could be obtained if a secondary corrosion resistant layer were employed in conjunction therewith. Further, this container structure suffers from the serious shortcoming that the structural strength and integrity of graphite is insufficient by itself to withstand severe mechanical loading conditions such as may be experienced in spacecraft applications. Further, if the graphite tube is made thick enough to provide structural support, it will exhibit high electrical resistance/impedance, which significantly adversely impacts the specific energy of the Na/S cell in which it is utilized. In another embodiment, the above-described cathode container structure is utilized as a liner within a metal container disposed in surrounding relationship thereto. Of course, this embodiment suffers from disadvantages and shortcomings similar to those discussed previously in connection with the Breiter and Damrow et al. cathode containers.
Based on the above and foregoing, it is clear that there presently exists a need for a cathode current collector/container for high energy density electrochemical cells, especially Na/S cells, affording greater corrosion protection and a longer service life than afforded by presently available Na/S cells, e.g., on the order of 10 years. More particularly, there is a need for a cathode current collector/container which overcomes the above-delineated shortcomings and disadvantages of presently available cathode current collector/containers utilized in Na/S cells or the like. The present invention fulfills this need.